A conventional combined cycle power plant generally includes one or more gas turbines, heat recovery steam generators (“HRSG's”), and a steam turbine. Because excessive steam pressures generated when the gas turbine is operating at high- or full-load can stress steam turbine components during steam turbine startup, traditional combined cycle power plant startup procedures require placing low load holds on the gas turbines and placing restrictions on gas turbine loading rates to control steam pressure during steam turbine startup.
Such holds and restrictions contribute to high gas turbine exhaust emissions during startup, increased startup and loading times, and increased fuel consumption during startup and loading. Thus, by holding gas turbines at low loads and loading rates, the gas turbines are forced to operate at low efficiency with high exhaust emissions during steam turbine startup and loading. Further, low gas turbine loads and loading rates cause the revenues generated by combined cycle power plants to be lower during steam turbine startup and loading.
Various strategies are known in the art for providing combined cycle power plant startup procedures which allow gas turbines and HRSG's to be operated under normal operating conditions during steam turbine startup and loading. For example, one strategy is to include additional piping and valves in combined cycle power plants, to lower steam pressures prior to admission to the steam turbine by routing excess steam to equipment designed to contain the steam, such as to a condenser. However, the addition of piping to a combined cycle power plant is expensive. Additionally, the space available for additional piping in a combined cycle power plant may be limited, and the installation of additional piping may be difficult. Further, the configuration of additional piping in a combined cycle power plant may cause large, uncontrollable pressure drops, which can damage the power plant.
Thus, an improved system and method for combined cycle power plant startup is desirable in the art. For example, a system and method for routing steam flow during steam turbine startup and loading, to lower steam pressures prior to admission to the steam turbine while allowing gas turbines and HRSG's to operate under normal operating conditions would be advantageous. Additionally, a combined cycle power plant startup system that uses existing power plant piping may be desirable. Further, a combined cycle power plant startup system that is inexpensive, easy to install, and controllable under all power plant operating conditions would be accepted in the art.